Continous forming of anodes for capacitors

ABSTRACT

A METHOD OF FORMING AN OXIDE FILM ON METAL STRIP COMPRISING FUNCTIONALLY DIVIDING A TANK CONTAINING A FORMING ELECTROLYTE INTO AN ENTRANCE, A FORMATION, AND AN AGING SECTION, AND CONTINUOUSLY PASSING A STRIP THROUGH THE TANK FROM THE GENERATION OF AN ANODIC OXIDE THEREON.

May 29, M LL s ET AL 3,736,237

CONTINUOUS FORMING OF ANODES FOR CAPACITORS Original Filed Sept. 26.

2 Sheets-Sheet 1 STRIP OUT STRIP IN STR IP OUT 1111rI/J r I y 1973 M.KALLIANIDES ETAL CONTINUOUS FORMING OF ANODES FOR CAPACITORS OriginalFiled Sept. 26 1967 STRIP IN 2 Sheets-Sheet 2 STRIP OUT il I STRIP OUT ll l [w ll United States Patent M 3,736,237 CONTINUOUS FQRMHNG 0F ANQDESFOR CAPACITORS Milton Kallianides, 52 Gifford St., Brocltton, Mass.02401, and Gerhart P. Klein, 45 Raymond, Manchester, Mass. 01944Application Dec. 10, 1969, Ser. No. 880,487, which is a continuation ofapplication Ser. No. 670,723, Sept. 26, 1967. Divided and thisapplication Feb. 19, 1971, Ser. No. 117,023

Int. Cl. C23b 9/02; B01d 3/00 US. Cl. 204-28 6 Claims ABSTRACT OF THEDISCLOSURE A method of forming an oxide film on metal strip comprisingfunctionally dividing a tank containing a forming electrolyte into anentrance, a formation, and an aging section, and continuously passing astrip through the tank from the generation of an anodic oxide thereon.

This application is a division of application Ser. No. 880,487, filedDec. 10, 1969, now US. Pat. 3,616,425 which is a continuation ofapplication Ser. No. 670,723, filed Sept. 26, 1967, which is nowabandoned.

Solid electrolytic capacitors are being employed instead of commonlyused aluminum capacitors for many applications for many reasons. One isthe size advantage. Units having a given capacitance rating can bemanufactured so as to be smaller than aluminum electrolytic capacitorshaving the same rating.

Generally, solid electrolyte capacitors are fabricated by pressing andsintering a film-forming metal such as tantalum, aluminum, niobium,titanium, zirconium, hafnium, and the like into a pellet having amultiplicity of intercommunicating voids. During the fabricationthereof, the pellet is subjected to anodization electro-formation bypassing a direct current from the pellet through an electrolyte in whichthe pellet is immersed, to a vessel containing the electrolyte. Thevoltage app-lied to the anode ranges from about 10500 volts depending onthe thickness of the anodized film desired. The higher the voltage thatis used, the thicker the resulting film and the lower the resultingcapacitance. The electrolyte solution may be an aqueous solution ofsulfuric and phosphoric acid, and the like. The pellet is impregnatedwith a solution of a material convertible to a semiconductive oxide suchas manganous nitrate, and heated in air at a suflicient temperature ofabout 200 to 400 C. to effect the pyrolytic conversion of the manganousnitrate covering the pellet and permeating the pores to a semiconductivemanganese dioxide. The anodization step and the pyrolytic conversionstep may be repeated as many times as is necessary in order to obtain apellet having the desired electrical characteristics. The manganesedioxide layer acts as the cathode contact for the capacitor. The cathodeis com pleted using conventional methods whereby an electricallyconductive layer is applied over the manganese dioxide layer.Preferably, a graphite layer is applied to the manganese dioxide beforethe remaining layers are applied. The capacitor is completed byattaching leads to the anode and to the cathode and encapsulating thecapacitor.

The present invention is concerned with the anodization step. Moreparticularly, the invention is concerned with method and apparatus forthe continuous generation of an anodic oxide on a moving carrier stripof foil or wire. The concept of the invention is particularly adaptableto the anodization of pressed and sintered porous tantalum pellets andthe tantalum foil which carries them, although the invention extendsbeyond this configuration to include 3,730,237 Patented May 29, 1973 theanodization of strips of tantalum without the pellets,

or to any of the strip materials previously noted.

A continuous manufacturing process wherein pellets are carried by anendless carrier strip requires a continuous forming process with controlover the rate of formation of anodic oxide films. In the case ofre-formation, after the application of manganese dioxide, the voltagehas to be increased at a predetermined rate. In the latter case, a smallamount of current is drawn by the moving strip as compared with the highcurrent densities required for formation.

The invention describes forming tanks for both the anodic oxidation ofbare metal and the re-formation of previously generated oxide of themetal (tantalum) in between manganese dioxide deposition cycles and atthe end of the deposition of manganese dioxide.

An object of the invention is to provide a method and apparatus for thecontinuous anodization of a strip of foil or wire with or without havingsintered pellets attached thereto.

Another object of the invention is to provide a tank for the electrolytethrough which the foil passes.

Another object of the invention is to provide a tank functionally havingan entrance-formation section, and an ageing section.

Another object of the invention is to provide a tank functionally havingan entrance section, a forming section, and an ageing section.

Still another object of the invention is the provision of a tank havingbafiie means to provide for the sections.

Yet another object of the invention is to provide a tank having acontoured bottom forming the sections.

Another object of the invention is to provide a means within theentrance section for preventing a current surge when the strip entersthe tank. Such current surge may cause damaged oxide films on the stripsurface.

Yet another object of the invention is to provide a tank wherein therate at which the anodic oxide film is generated on the moving strip inthe forming section of the tank is constant. With the rate beingconstant, the growth rate of the anodic oxide is predictable anduniform.

Another object of the invention is to provide a tank wherein the bafilemeans or the tank bottom forms a geometry which is particularlyadaptable to formation of the anodic oxide film on bare metal.

An additional object of the invention is to provide a tank wherein thebattle means or the tank bottom forms a geometry which is particularlyadaptable to reforming the anodic-oxide film to repair any damage thatmay have occurred to the formed film.

With the above and other objects in view, which will appear as thedescription proceeds, this invention resides in a. novel method andapparatus for continuously anodizing strips or strips carrying sinteredanodes substantial ly as described herein and more particularly definedby the appended claims, it being understood that such changes in theprecise embodiment of the invention here disclosed may be made as comewithin the scope of the claims.

In the drawings:

FIGS. 1 and 2 are cross sections of an electrolytic tank schematicallyshowing a baflie means for functionally dividing the tank; and

FIGS. 3 and 4 are cross sections of such a tank schematically showingthe functional division of the tank formed by the tanks configuration.

Generally speaking, the objects of the invention are accomplished byproviding a tank for holding an electrolyte through which a metal stripsuch as tantalum is passed to anodize the strip. Means are provided tofunctionally divide the tank into at least two sections: an entrance and3 a formation section, and an ageing section. The formation sectiongives a constant formation rate by the arrangement of the means formingthe sections. Means are provided in the entrance section to prevent theoxide formation until the strip reaches the formation section thusavoiding a current surge.

In one embodiment of the invention, the means divid ing the tank intoits functional sections comprises, in general, the contour or theconfiguration of the bottom of the tank. In another embodiment bafflemeans are disposed within the tank.

Referring now to FIG. 1, there is shown an electrolytic bath 10 whereinan electrolyte 11 is contained in an open top tank 12. A suitableelectrolyte would be an aqueous solution of sulfuric or phosphoric acid,and the like. The tank needs to be constructed from electricallyinsulating materials, such as Teflon, polypropylene, and others, or ifit is made from stainless steel, it has to be provided with aninsulating liner both on the inside and the outside. The outerinsulating coating is required for safety reasons since the tank cannotbe grounded and will carry a potential equal to the formation voltage(up to 500 v.). The tank is functionally divided into three sections: astrip entrance section A, forming section B and an ageing section C, thesections being formed by baffle means 13. The strip 14 that is fedthrough the tank is, as shown, at ground potential when it enters thetank. The strip is guided through the tank by suitable pulleys 15.Electrical contact from a power supply (not shown) is provided bysuitable rollers or brushes 16. As shown in the drawings, there are nosintered powder pellets attached to and carried by the strip. However,as previously noted, it is within the scope of the invention to have thestrip carry the sintered pellets, with the pellets being anodized alongwith the strip, as the strip passes through the electrolyte.

An active auxiliary electrode 17 is disposed within the entrance sectionA, the electrode being at ground potential. The electrode is constructedof a material such as a one square inch piece of platinum foil. Thisauxiliary electrode serves the purpose of preventing formation of oxideon the strip before the strip enters the formation section B. It willcarry current only in cases where the resistivity of the electrolyte isnot properly matched with the speed of the strip and the specificforming charge thus preventing a current surge. A counter electrode(cathode) 18 is disposed within the ageing section C. Electrode 18 couldbe constructed of stainless steel, for example. It substantiallysurrounds the whole of the ageing section so that the potential in theageing section is constant.

The rate of formation of anodic oxide films is directly proportional tothe current density at the surface of the strip. In the case of aconstant rate of formation, the current density over the full length ofthe strip in the forming section is constant. Thus, for a tank of agiven width, and for an electrolyte of a given conductivity, the heightof the forming section, or the depth of the electrolyte, has increasedlinearly from the beginning of the forming section to its end, for aconstant rate of formation. As shown, the increasing depth isaccomplished by surface 13a of baiile means 13. The geometry of thebaffle means 13, once fixed, is not readily changed. Thus, in order tobe able to process strips of diiferent forming voltage at differentrates of formation, one can adjust the conductivity of the electrolyte.Alternatively, bafl'le means 13 can be made readily adjustable.Likewise, where necessary, forming rates other than linear can bederived by providing a non-linear rise in surface 13a.

A plurality of pulleys 15 are provided in ageing section C in order tomore readily increase the length of strip being aged.

The length of the forming section B and the number of passes in theageing section C depend on the speed of the strip 14, the forming rate,and the ageing time. Typical forming rates for tantalum anodes-Withtantalum pellets carried by the strip-would be in the range of -20 Voltsper minute. In a typical example, the forming rate would be 10volts/min., the speed of the strip 7.5 cm./ min., the target voltage 150volts, and the ageing time 1 hour. This requires section B to be 112.5cm. long, and 450 cm. of strip to be aged in section C. The resistivityof the electrolyte would be 150 ohm-cm., and the slope of the bafflesurface 13a would be about 1 to 1.

FIG. 2 illustrates the tank with the baffle means 13 being arranged forre-forming. In the case of the re-formation of tantalum oxide followingpyrolysis, the oxide film is already there, but it may have becomedamaged in places and thus requires careful re-app-lication of voltagein order for the damage to repair itself. The application of voltageshould be programmed in such a way as to prevent additional damagethrough sudden application of full voltage. There should preferably be alinear increase of voltage with time. The amount of voltage required issubstantially less than in the formation of a bare strip.

Again the tank comprises three sections: an entrance section A, areformation section B, and an ageing section C, the sections beingformed by baflie means 13. The strip 14 entering the tank is grounded asbefore. Constant voltage is applied between the counter electrode(cathode) 18 and the auxiliary electrode 17. Sections A and C areconnected by a channel 19 formed by bafiie surface 13b and the bottom ofthe tank. The electrical resistance of the channel-that is, there-forming section of the tankis constant. A constant current rflows,thus setting up a constant potential gradient across the length of thechannel. The resistivity of the electrolyte for re-formation is in thekilo-ohm range, the cross section of the channel of the order of 1 cm.the length on the order of cm., and the current flowing through thechannel is of the order of 1 ma. The currents required for re-formationare usually of the order of microamperes and, therefore, small comparedto the current flowing in the channel. This arrangement is equivalent toa potentiometric set up in which the strip consumes the charge requiredto lift it to the needed potential without significantly affecting thechannel current.

The auxiliary electrode in section A now plays an essential role in thatit defines the point at which the potential starts to rise. If thechannel can be compared with the slide wire of a voltage divider, thenan increment of the strip (e.g. 1 cm.) can be compared to the slidingcontact on the slide wire and the process of re-formation with themovement of the strip increment from one end of the channel (slide wire)to the other. The rate at which the strip increment travels from one endto the other determines the rate of re-formation. Ageing of the stripafter re-formation is done in an analogy after forming.

Referring now to FIGS. 3 and 4, there is shown alternate baths 10a and10b for a different form of the invention. In the embodiments of thisform of the invention, the means to functionally divide the tanks 12aand 12b into the afore-mentioned sections comprises the contour orconfiguration of the bottoms of the tanks. The tank shown in FIG. 3 isfor the anodic formation of bare metal, and as such it is similar to thetank of FIG. 1. As shown, the bottom of the tank has a first horizontalportion 30, a sloping portion 31 and a second horizontal portion 32, thesloping portion 31 yielding an increasing depth of the electrolyte 11for the formation section B. It is also noted that the strip 14 is beingfed through the formation at angle off the horizontal. The magnitude ofsuch angle is dependent upon the slope of bottom portion 31 and theformation rate desired, the rate once determined being constant. As inthe case of FIG. 1, the auxiliary electrode 17 prevents oxide formationon the strip until the strip enters formation section B.

FIG. 4 is comparable to FIG. 2; that is, the tank is designed for stripre-formation. The bottom of the tank is now formed in two levels byhorizontal portions 40 and 41. As in the case of FIG. 2, bottom portion40 forms a channel 19a, the channel being used to re-form the strip asis done in FIG. 2.

It is to be understood that the tank and the bafiie means of FIGS. 1 and2 would be fabricated from an electrically insulating material, or atleast the surfaces in contact with the electrolyte, including pulleys15, need to be of an insulating material such as Teflon or other similarmaterial. In addition, means for heating, thermostatic control of thetemperature, circulation of the electrolyte and other equipment known bythose skilled in the art need to be incorporated with the tank.

Thus there is described a novel method and apparatus for anodizing metalstrip in a continuous manner, the novel features of which would bereadily apparent to those skilled in the art. In addition, the presentinvention is not intended to be limited to the disclosure herein, andchanges and modifications may be made in the disclosure by those skilledin the art without departing from the spirit and scope thereof. Suchmodifications and variations are to be considered within the purview andscope of this invention and the appended claims.

We claim:

1. A method of forming an anodic oxide film on metal strip comprising:

providing a tank containing a liquid electrolyte;

passing the strip to be oxidized beneath the surface of the electrolyte,said strip being at substantially ground potential;

applying a formation voltage to said tank;

dimensioning said tank so that the distance between said strip and saidtank so that the depth of the electrolyte increases substantiallylinearly from the beginning of the formation section to its end. 2. Amethod according to claim 1 in which a bafile is provided to vary thedepth of the electrolyte.

3. A method according to claim 1 in which the bottom of the tank variesto vary the depth of the electrolyte.

4. A method according to claim 1 in which said metal is a film-formingmetal.

5. A method according to claim 1 in which after the formation operationthe strip is aged.

6. A method according to claim 1 in which said strip carries sinteredpellets.

References Cited UNITED STATES PATENTS 2,370,973 3/1945 Lang 204-28 X2,901,412 8/1959 Mostovych et a1. 204-211 3,079,308 2/1963 Ramirez et al204-28 FOREIGN PATENTS 467,024 6/1937 Great Britain 204-28 F. C.EDMUNDSON, Primary Examiner U.S. Cl. X.R. 204-5 8 UNITED STATES PATENTOFFICE (1E ET I F [(1 ATE F CORRECTION Patent NO- 3,736,237 Dated ma 99'mm Invent0r(s) Milton Kallianides & Gerhart P. Klein It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Insert after the inventors: Assignors to P. R. Mallory & Co. Inc. v

Col. 3, line 24, after "A," insert ------a- C01. 3, lines 558256, 'after"has" delete "increased" and insert --has to increase--:. 1

Signed and sealed this 27th day of November 1973 (SEAL) Attest:

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer ActingCommissioner of Patents ORM PO-10 50 (10-69) USCOMM-DC eoavs-pgs us.GOVERNME mmmqs ornoz: agis o-asg-ssz

